1. Field
This application relates generally to wireless communication and more specifically, but not exclusively, to maintaining virtual active sets.
2. Introduction
A wireless communication network may be deployed over a defined geographical area to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within that geographical area. In a typical implementation, access points (e.g., corresponding to different cells) are distributed throughout a network to provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the geographical area served by the network.
In general, at a given point in time, the access terminal will be served by a given set of one or more cells. Over time, the signal quality seen by the access terminal may change, whereby the access terminal may be better served by a different set of one or more cells. In such a case, to maintain mobility for the access terminal, the access terminal may be handed-over from its serving cell set to the other cell set.
To facilitate this mobility, the network may instruct the access terminal to measure signals (e.g., beacon/pilot signals) from cells on the current serving frequency and on other frequencies. The network then uses the signal quality of the measured signals to determine whether the access terminal should remain on its current serving cell set or switch to another cell set. These measurements may be periodic or event-triggered. As an example of the latter case, the network may configure an access terminal with one or more parameters (e.g., a threshold) and instruct the access terminal to send a measurement report whenever the measured signal quality meets the criteria specified by the parameter(s) (e.g., signal quality exceeds the threshold).
In some implementations, a network may support soft handover of an access terminal. In such a case, the access terminal maintains concurrent connections (e.g., radio links) with multiple cells on the serving frequency. The cells with which the access terminal maintains these connections are referred to as the active set.
As mentioned above, apart from the serving frequency, there are other frequencies that may be available to an access terminal. Consequently, an access terminal may maintain other active sets corresponding to these other frequencies. Each of these active sets is referred to as a “virtual” active set (VAS) because an access terminal generally does not actively maintain connections with the cells listed in the VAS. Rather, the VAS on a particular frequency, e.g., frequency j, is generally understood to be the active set that is expected to be used if the access terminal were to be camped on the frequency j. A single VAS is defined for a given one of these other frequencies.
The VASs are used in conjunction with the active set to determine whether to leave an access terminal on its current serving frequency or to hand the access terminal over to one of the other frequencies. For example, the access terminal may measure certain signal quantities (e.g., Ec/Io) from the cells listed in the active set and each VAS and generate a corresponding quality estimate for each frequency based on an average of the measured signal quantities. From the resulting set of frequency quality estimates, a determination may be made as to which frequency will provide the best service for the access terminal. An example of a VAS is described in the 3GPP document TS 25.331 at Section 14.11.
As the demand for high-rate and multimedia data services rapidly grows, there lies a challenge to implement efficient and robust communication systems with enhanced performance. To supplement conventional network access points (e.g., macro access points), small-coverage access points may be deployed (e.g., installed in a home or workplace) to provide more robust indoor wireless coverage or other coverage to access terminals. Such small-coverage access points may be referred to as, for example, femto access points, femto cells, Home NodeBs (HNBs), Home eNodeBs (HeNBs), or access point base stations. Typically, such small-coverage access points are connected to the Internet and the mobile operator's network via a DSL router or a cable modem. For convenience, small-coverage access points may be collectively referred to as HNBs (or HNB cells) in the discussion that follows.
Access points such as HNBs may be configured to support different types of access modes. For example, in an open access mode, an access point may allow any access terminal to obtain any type of service via the access point. In a restricted (or closed) access mode, however, an access point may only allow authorized access terminals to obtain service via the access point. For example, an access point may only allow access terminals (e.g., so called home access terminals) belonging to a certain subscriber group (e.g., a closed subscriber group (CSG)) to obtain service via the access point. In a hybrid access mode, alien access terminals (e.g., non-home access terminals, non-CSG access terminals) may only be allowed to obtain access via the access point under certain conditions. For example, a macro access terminal that does not belong to a HNB's CSG may be allowed to access the HNB only if the HNB is not currently serving a home access terminal. For convenience, a cell (e.g., a HNB) that is associated with one or more CSGs may be referred to as a CSG cell in the discussion that follows.
In practice, various problems may arise in a system that employs HNBs (CSG cells) and VASs. These problems may arise in a co-channel deployment where HNBs and one or more macro cells operate on the same frequency or in a dedicated channel deployment where HNBs operate on a different frequency (e.g., a dedicated frequency) than macro cells.
In a co-channel deployment, false event trigging may occur for an access terminal that is not interested in HNB coverage (hereafter referred to as a macro access terminal, for convenience) or for an access terminal that is interested in HNB coverage (hereafter referred to as a HNB access terminal, for convenience). This false triggering may occur as a result of HNBs and macro cells being included in the VAS. In this case, a frequency quality estimate based on this VAS may be incorrect.
For example, if there is poor macro cell quality but good HNB quality on another (non-serving) frequency, a relatively high overall frequency quality estimate may be indicated for this other frequency. Thus, a macro access point that is only interested in macro coverage may be handed-over to the other frequency where the macro coverage is poor (e.g., worse than on the serving frequency).
Conversely, if there is good macro cell quality but poor HNB quality on another (non-serving) frequency, a relatively high overall frequency quality estimate may again be indicated for this other frequency. However, a HNB access terminal that is only interested in HNB coverage may be handed-over to the other frequency where the HNB coverage is poor (e.g., worse than the macro coverage on the serving frequency).
If HNBs are not included in a VAS to avoid the adverse impact on macro access terminal mobility discussed above, a HNB access terminal may not be handed-over to a frequency with HNBs in cases where such a handover is desired. For example, if there is poor macro cell quality but good HNB quality on another (non-serving) frequency, the HNB access terminal may be left on the current serving frequency because a poor frequency quality estimate would be reported for the other frequency.
In a dedicated channel deployment, various problems may arise in conjunction with HNB access. For example, a frequency quality estimate based on multiple HNBs may be incorrect. Here, if there is poor quality for a user's HNB (e.g., a HNB associated with a CSG) but high quality for a neighbor HNB, a relatively high overall frequency quality estimate may be indicated for the dedicated HNB frequency. Thus, a HNB access terminal that is only interested in its home HNB coverage may be handed-over to the HNB frequency in a case where the home HNB coverage is poor (e.g., worse than the macro coverage on the current serving frequency).
As another example, if collectively the quality appears high for enterprise-based HNBs (e.g., HNBs provided at a work site), but on an individual basis the quality of each HNB is poor, a relatively high overall frequency quality estimate may be indicated for the dedicated HNB frequency. Thus, a HNB access terminal may be handed-over to the HNB frequency in a case where the HNB coverage is actually poor (e.g., worse than the macro coverage on the current serving frequency).
Moreover, the cells that are allowed to be included in a VAS for an access terminal are typically limited to the cells that are specified by the network. For example, the network may send an Intra-frequency cell info list and an Inter-frequency cell info list that specify the cells that the access terminal may include in a CELL_INFO_LIST or a neighbor cell list (NCL). However, this limits the number of primary scrambling codes (PSCs) that may be used by the HNBs on the dedicated frequency. Given that there may be a relatively large number of HNBs deployed on the dedicated frequency (e.g., hundreds or more), this may increase the probability of PSC confusion on the dedicated frequency.